During processing of silver halide color photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions are desirably acidic, and contain complexes of high valence metal ions, such as ferric ions, and various organic ligands. Both the metal ions and the chelating ligands present environmental concerns and are the subject of increasing regulatory scrutiny. Thus, a primary desire in this industry is to design bleaching compositions that are more compatible with the environment, with reduced use of high valent metal complex bleaching agents.
Bleaching solutions containing peroxide, or peracids such as persulfate, perborate, perphosphate, percarboxylic acid or percarbonate as bleaching agents offer an alternative to the ferric complex bleaching solutions. They are less expensive and present lower chemical and biological demands on the environment since their by-products can be less harmful. A number of such compositions have been explored, but to date they have not found wide acceptance in the trade for various reasons.
Various peroxide and peracid bleaching solutions are described, for example, in U.S. Pat. No. 5,318,880 (English et al), U.S. Pat. No. 5,464,728 (Szajewski et al), U.S. Pat. No. 5,508,151 (O'Toole et al) and U.S. Pat. No. 5,521,056 (Buchanan et al).
While persulfate bleaching agents have low environmental impact, they have the disadvantage that their bleaching activity is slow and thus requires the presence of a bleaching accelerator or the use of an accelerator prebath, as widely practiced to process motion picture films. The most common bleaching accelerators are thiols that have offensive odors.
Because hydrogen peroxide reacts and decomposes to form water, a peroxide based bleaching solution offers many environmental advantages over persulfate and ferric complex bleaching solutions. As a result, many publications describe peroxide bleaching solutions, including U.S. Pat. No. 4,277,556 (Koboshi et al), U.S. Pat. No. 4,301,236 (Idota et al), U.S. Pat. No. 4,454,224 (Brien et al), U.S. Pat. No. 4,717,649 (Hall et al), U.S. Pat. No. 5,550,009 (Haye et al), U.S. Pat. No. 5,541,041 (Haye), U.S. Pat. No. 5,641,615 (Haye et al), WO-A-92/01972 (published Feb. 6, 1992), WO-A-92/07300 (published Apr. 30, 1992) and EP 0 428 101A1 (published May 22, 1991). These compositions may comprise various amounts of chloride ions and have a pH in the range of 5 to 11.
Hydrogen peroxide bleaching compositions, however, often cause physical defects in the processed photographic elements, such as blistering (or vesiculation), and suffer long term stability problems at certain acidity levels. In addition, peroxide is ineffective in oxidizing developed silver to silver halide at acidic pH values, and thus it cannot be used to directly replace current high valent metal ligand bleaching solutions.
There is a desire and need in the photographic industry to find solutions to all of these problems. Thus, there is a need to avoid the use of bleaching accelerators and high valent metal bleaching agents while providing an effective acidic bleaching environment.
In copending and commonly assigned U.S. Ser. No. 08/947,072, filed on Oct. 8, 1997, noted above, useful acidic periodate bleaching compositions and methods of bleaching are described. However, such methods include the use of an acidic prebath immediately before bleaching.
The purpose of this acidic prebath is two-fold. Firstly, it serves as a wash solution to remove any residual color developing agent carried over by the processed element from the color developer solution. Secondly, it serves to lower the pH within the element layers to a level at which any incorporated dye forming color couplers are no longer reactive with oxidized color developing agent. Without the prebath before bleaching, the processed element would carry color developing agent into the bleaching solution, and this color developing agent would be oxidized by bleaching agent. Additionally, incorporated dye forming color couplers may still be reactive with oxidized color developing agent, forming additional dye. Since this dye formation would occur in a non-imagewise fashion, the newly formed dye would be seen as undesirable overall dye "stain" in the processed photographic element.
Yet, there remains a need in the art to eliminate the use of the acidic prebath so as to reduce the number of steps to be followed and solutions to be prepared and maintained. Thus, prebath elimination would increase processing time and productivity in a processing facility.
In order to process such photographic elements without the use of an acidic prebath, the bleaching solution must be sufficiently oxidizing to oxidize developed silver to a silver salt, but acidic enough to deactivate the reactive dye forming color couplers in the elements before they can react with color developing agent carried over from the color developer solution. With relatively weak oxidizing bleaching agents, such as chelated high valence metal ions (for example, the common ferric chelates of aminocarboxylic acids), bleaching compositions may be formulated over a wide range of acidity and bleaching agent concentration for silver oxidization without unwanted dye stain. These expectations can be characterized as the need to reduce residual silver in a processed element to below about 0.04 g/m.sup.2, and to minimize dye stain, or minimum density (Dmin) in unexposed areas. Acceptable Dmin would be that obtained from comparable processing of the same element using an acidic prebath prior to the use of a conventional chelated iron bleaching agent.
When much stronger oxidants are used as bleaching agents, however, we discovered that there is less flexibility in formulating a bleaching composition that performs all of the desired functions without dye stain. For example, merely using the periodate bleaching compositions and methods described in our copending U.S. Ser. No. 08/947,072, noted above, in the absence of an acidic prebath, did not provide desired results. Bleaching was effective in most cases, but dye stain was unacceptably increased. Thus, there is a need to find a way to use a periodate bleaching composition in the absence of acidic prebath solutions.